Believing that an evil spirit is trying to obsess one is a dangerous belief, and when one comes to believe he is obsessed by an evil spirit, though there is not an evil spirit within a thousand miles of him, he will have all the symptoms.

Intensely preoccupiedwith or by a given topic or emotion; driven by a specified obsession.

1997, Philip Roth, American Pastoral:

What was starting to unsettle him, to frighten him, was the idea that Merry was less horrified now than curious, and soon he himself became obsessed, though not, like her, by the self-immolators in Vietnam but by the change of demeanor of his eleven-year-old.

1999, Mark Lawson, The Guardian, 28 Jun 1999:

Strangely, although it is an international cliché that the British are obsessed with the weather, it is a fixation with minor irritations: will rain spoil the wedding, the Test Match, the bank holiday?

2007, Thomas Pynchon, Against the Day:

Everyone lay around in a sort of focused inertia, drinking, handing cigarettes back and forth, forgetting with whom, or whether, they were supposed to be romantically obsessed.